Gratian sent Arbogast to Theodosius to help him against the Goths in Thrace in 380. In 384, Arbogast recovered Gaul from the usurper Magnus Maximus (whose Gothic troops had killed Gratian) for the Western emperor who had succeeded Gratian, Valentinian II. Arbogast served as magister equitum under Valentinian II. Arbogast imposed a treaty on the Franks and had become quite powerful. When Valentinian tried to remove him from power, in 391, Arbogast ignored the order, stating that only Theodosius had such authority. Valentinian II died shortly thereafter, May 13, 392, supposedly, a suicide.
Arbogast proclaimed a pagan, Eugenius, the magister scrinii, Emperor in the West. In 394, Theodosius marched west to deal with both the usurper and the restoration of paganism. At the Battle of the Frigidus River (now, Vipava River), in the Alps, two days of fighting resulted in the victory of Theodosius, the beheading of Eugenius the usurper, and the suicide of Arbogast.
Sources include the Encyclopedia Britannica